In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to control circuitry in a control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all the devices used in the measurement, control and monitoring of industrial processes.
Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration, mechanical shock, et cetera. These field devices can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
Some field devices include a transducer. A transducer is understood to mean either a device that generates an electrical output based on a physical input or that generates a physical output based on an electrical input. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow sensors, positioners, actuators, solenoids, indicator lights, and others.
Typically, each field device includes communication circuitry that is used for communicating with the process control room, or other circuitry over the process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device.
Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device being connected to the control room by a single two-wire control loop. Typically, a voltage differential is maintained between the two wires within a range of voltages from 12-45 volts for analog mode and 9-50 volts for digital mode. Some analog field devices transmit a signal to the control room by modulating the current through the current loop to a current that is proportional to a sensed process variable. Other analog field devices can perform an action under the control of the control room by controlling the magnitude of the current through the loop. Some process control loops also carry digital signals for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Moreover, digital field devices do not require separate wiring runs for each field device. Further, field devices that communicate digitally can respond to and communicate selectively with the control room and/or field devices.